Observation of a Discrete-Line Superdeformed Band up to60ℏinDy152

Abstract

A rotational band of nineteen transitions with a moment of inertia 3$ nA of 84£ 2 MeV" 1 has been observed in 152 Dy. The band feeds into the oblate yrast states between 19" and 25" and it is proposed that the lowest member of the band has a spin of 22 + and thus the band extends up to 6Qfr. It is identified as the yrast superdeformed band and its intensity accounts for the whole of the ridge structure seen previously in continuum E y -E r correlations.PACS numbers: 21.10. Re, 23.20.Lv, 27.70.+q The nucleusDy has been extensively studied and three different structures have been identified. The low-spin yrast levels have a pseudovibrational structure 1 which develops into a low-deformation (fi = 0.15) prolate rotational band 2 extending up to 40fr. This band, in the spin region between SR and 38£, lies between 0.5 and 1.5 MeV above the yrast states which have a weak oblate structure formed by particles in equatorial orbits. 3 " 5 At higher spins the y-ray continuum is dominated by a collective E2 bump. 6 Part of this bump has been shown to arise from superdeformed (/J^O^) bands from the existence of ridges with a moment of inertia3 (2) = (85 ±2)H 2 MeV" 1 in E y 'Ey correlation spectra. 7,8 In this Letter we present data showing a discrete-line rotational band extending over nineteen transitions from 602 to 1449 keV with an almost constant energy separation of 47 keV which corresponds to the superdeformed moment of inertia. The major -y-ray decay deexciting the band feeds into the yrast oblate structure between the 19"" and 25"" states and then proceeds via the 60-ns 17 + isomer. Additionally 25% of the decay intensity bypasses this isomer. We propose that the decay process from the bottom of the band is essentially statistical, involving several transitions, and we assign the spin at the bottom of the band to be 22£, thus establishing the spin at the top of the band to be 60fr. This is the first observation of a discrete-line superdeformed band and it extends the spin at which discrete states have been seen from about 46* (e.g., 158 Er, Tj0m etaL 9 ) to 60T.The experiment was carried out on the tandem accelerator at the Daresbury Laboratory using the TES-SAS spectrometer, which consists of a 50-element bismuth germanate (BGO) crystal ball similar to that used in TESSA2 10 with twelve escape-suppressed germanium detectors. 11 The states in 152 Dy were populated by the reaction 108 Pd( 48 Ca,4>7) at 205 MeV with a target consisting of two 500-/ig-cm~2 self-supporting foils isotopically enriched at 95% in 108 Pd. A 15-mgcm" 2 gold catcher foil was positioned 5 cm downstream of the targets such that it was outside the focus of the germanium detectors but within the full detection efficiency of the BGO ball. A total of over 150 million double (Ge-Ge) coincidences were recorded together with the sum energy and number of hits (fold) in the BGO ball. The time difference between the BGO ball and the second-coincidence germanium detector was recorded and enabled most of the neutron-induced events in the germanium detec...

Spin alignment in superdeformed rotational bands

“…This nucleus may represent a double-closed shell for superdeformed shapes, or its central position may be only apparent While the zero-alignment position does depend on which of these bands is taken as reference, the main conclusions do not The incremental alignments of these eight bands are plotted in Fig. 7, where we have also included the excited (*) superdeformed bands, I53Dy* [21] and 151Tb*[l], referred to the band in 152Dy [22]. It is apparent that the data cluster around the lines drawn at± 1, ± 0.5, and 0, although almost all the values for bands in the mass-190 region decrease below frequencies of about 0.2 MeV, and the values for one 191Hg* band decrease at the highest frequencies.…”

Observation and Quadrupole-Moment Measurement of the First Superdeformed Band in theA∼60Mass Region

“…In recent years the study of superdeformed (SD) bands has been one of the most exciting topics in nuclear structure research. Since the first observation of highspin superdeformation in 152 Dy [1], SD bands have been identified in many nuclei in the A ϳ190, ϳ150, ϳ130, and ϳ80 mass regions [2]. Based on calculations of large SD shell gaps in the single-particle energy levels for particle numbers N, Z ഠ 30 32 [3,4], SD bands have also been predicted in the A ϳ 60 mass region [5].…”

Superformed Bands in Atomic Nuclei

“…A real turning-point was, however, the direct observation of a discrete SD rotational band in 152Dy by the Daresbury-Liverpool-Copenhagen group in 1986 [5] using TESSA3. The y-ray spectrum of the band is shown in Fig.…”